The study of the dynamics of cell processes is necessary for understanding cell function, organism development, and disease. Increasingly, control engineers and cell biologists are collaborating as they recognize similar attributes in the systems they study, such as amplification, positive and negative feedback, regulation and control, oscillatory and nonlinear behavior, bi-stable behavior, disturbance rejection, noise rejection, and robustness. In many instances, the vocabularies of the engineering and biology domains are the same with regard to dynamic systems. However, one significant difference between the two domains is that there is generally no separate control unit in a biological regulation process. Rather, feedback control of a reaction in a biochemical pathway is performed implicitly by biological agents downstream of the reaction which manipulate the activity of other biological agents upstream of the reaction.
Microfluidics techniques are ideally suited to creating and maintaining the types of external chemical gradients that generate the behavior just mentioned. The ability to determine the functioning of a single cell has been handicapped by the absence of technology to introduce spatiotemporal stimulation to localized subcellular domains within single cells of sub-micron specificity. Patch clamping, micropipetting, and laser microsurgery are useful for examining local domains, but none of these methods have the potential to be as robust as those enabled by a micro- and nano-technological approach. Alternative fabrication techniques at the organic-inorganic interface can regulate the attachment and spreading of individual cells, and fluidic devices have been implemented to mediate cell population attachment, as well as to deliver chemical reagents to specific cell populations. Many cellular characteristics and processes have been discovered to be spatially and temporally responsive, including cell structure, motility, and apoptosis. To measure internal cell responses, methods for working at subcellular levels with control over these gradient behaviors are needed. Micro- and nano-technological approaches are ideally suited to these applications, as they can be used to design and develop systems on the size scale of cells and molecules and have been successfully interfaced with the cellular and molecular worlds in areas such as DNA transport, drug delivery targeting for cancer treatments, and electrically stimulating neural cells.
In U.S. Patent Application Publication No. 2008/0131323, published on Jun. 5, 2008, and titled “Method and Apparatus Utilizing Laminar Flow Interface Control In A Microfluidic Device,” the present inventors described the design, fabrication, testing, and operation of apparatuses, systems, and methods for controlling the position of the interface between two or more laminar flow streams in a microfluidic network. Those apparatuses, systems, and methods allow researchers to study the behavior of cells and other objects as “black box” systems, responding to input signals in observable ways to generate output signals that can include cell position or chemical concentration. For example, variations of a chemical or other environmental factor of a cell can constitute an “input,” and the cell's response to these inputs can constitute an “output.” The previous invention may be used to study fundamental dynamic responses of cells, including threshold response and frequency response.
In the '323 publication, control of the position of the interface between two or more laminar flow streams is achieved using a closed-loop system that regulates pressure instead of flow. That approach achieves a high precision of positioning of the interface even at very low flow rates of one or more streams. In one embodiment of the '323 publication, the pressure in the closed-loop system is controlled using a direct current motor to actuate a syringe plunger. While that embodiment achieves the goal of providing precise control over the interface between two or more laminar flow streams, it has a limitation in the relatively small volume of the syringes used in the pressure control system. An implication of this limitation is that it constrains the duration of studies that can be conducted with the system.